1. Field of the Invention
This invention relates to a process for manufacturing a twisted structure of a fiber-reinforced synthetic resin material, the structure having high tensile strength and flexibility and being suitable for use as a tension member for a non-metallic optical cable or the like.
2. Description of Prior Art
In recent years, it has been deemed important to render communication cables free of induction and lighter in weight, and there has been a conspicuous trend toward the use of non-metallic cables in place of metallic cables. As a result, rod-shaped members made of fiber-reinforced synthetic resin material (hereinafter referred to as "FRP") with glass fibers or Aramid fibers have been used widely as tension members in cables.
However, an attempt to obtain a rod-shaped FRP member having a tensile strength comparable to that of metallic tension members (steel wires, stranded steel wires, etc.) involves use of a member having a larger outside diameter, resulting in the cables obtained having an excessively high stiffness with poor ability to be handled.
To overcome these problems, it has been proposed to twist a plurality of cured rectilinear FRP rods together so as to obtain flexibility while retaining a sufficient tensile strength. Such proposal, however, has proved that it is difficult to obtain a stable twisted structure because the strands constituting the structure tend to repel each other, thereby loosening the twist.
Also during the production of the twisted structure, it is necessary to wind the cured FRP rod as the strands on bobbins and to supply the rod while rotating the bobbins, which leads to a complicated stranding step and a high cost.
On the other hand, for obtaining a FRP rope member, there has been known a method in which reinforcing fibers are impregnated with an uncured thermosetting resin, the impregnated fibers are formed to have a predetermined outside diameter followed by coating with a thermoplastic resin to obtain a composite strand, and such strands are twisted together prior to the curing of the thermosetting resin.
The conventional method of manufacturing the above rope member by use of the composite strands, however, has had the following problems. Since the twisting of the strands and the curing of the thermosetting resin are carried out discontinuously in separate steps from the production of the uncured composite strands, difficulties occur regarding the storage stability of the uncured thermosetting resin and, further, offset or disorder of the arrangement of the reinforcing fibers in the composite strands tends to be caused during take-up of the strands of drums, bobbins, or the like, with lowering of physical properties after curing.
Accordingly, it is an object of the present invention to provide a process for manufacturing a twisted FRP structure having sufficient flexibility required in practical use as well as satisfactory strength.
It is another object of the invention to provide such a process which enables efficient manufacture of the FRP structure.